1. Field
The present disclosure relates to the field of tires comprising a cellular material.
2. Description of Related Art
In order to improve the grip of a tire, document WO 2009/003576 or WO 2009/003577 provides a tire comprising a tread that has                an outer, rubber layer, subsequently referred to as a running layer, made of cellular material, in which hollow tread pattern elements are made, and        an inner rubber layer having a hardness greater than that of the outer layer.        
In order to manufacture such a tire, an uncured blank is produced comprising a mass of rubber, intended to form the running layer, in which a pore-forming agent is incorporated. Next, the blank is placed in a vulcanization mold and the blank is heated under pressure in this mold.
Under the effect of heat and pressure, a chemical reaction involving the pore-forming agent creates gas bubbles that form cells in the material, which therefore becomes cellular. The outer contour of the running layer is set by the mold. Once curing is complete, the tire is extracted from the mold.
In order for the grip performances to be advantageous, it is desirable for the degree of expansion of the cellular material to be greater than or equal to 50%, which requires a minimum amount of pore-forming agent in the mass of rubber intended to form the running layer.
With such a degree of expansion, after extracting the tire from the mold, it is observed that the shape of the running layer continues to evolve, so that this layer expands radially beyond the desired outer contour. This expansion is the result of the following two effects.
On the one hand, on leaving the mold, the volume of the cells containing the gas continues to increase, swelling the running layer beyond the contour set by the mold.
On the other hand, due to the difference in pressure between the moment when the tire is in the mold and the moment when the tire is taken out of the mold, an expansion of the running layer is observed on leaving the mold.
These two effects do not make it possible to obtain, on leaving the mold, a precise geometry of the running layer and therefore do not make it possible to obtain the desired performances of the tire.
Moreover, the gases dissolved in the cellular material have a tendency to accumulate within bubbles of relatively large size. These accumulated bubbles form large-sized cells that weaken the running layer and degrade the performances of the tire.